Subphylum Vertebrata

The largest taxon within the phylum Chordata is the subphylum Vertebrata. Of all the animals they have the biggest ecological impact on the environment. Although in quantity they do not possess the highest number of individuals per species, they include the largest animals (Cetacea), and the largest average-sized species. The mass of whales is only exceeded by the mass of the largest trees.

All vertebrates possess at least a vertebral column, a braincase including a complex brain associated with specialized sense organs, usually paired limbs, a skeleton of bone and/or cartilage, an epidermis plus dermis, a kidney, liver, and pancreas, a complex heart with at least three chambers and the blood has corpuscles containing haemoglobin, and a neural crest. The vertebrates developed after the Protochordata (lacking cranium, vertebral column and specialized anterior sense organs) invaded freshwater. The radiation of the vertebrates to tetrapods occurred in freshwater. So marine lineages of vertebrates migrated from freshwater to the oceans and are dead-end groups in terms of evolution.

The subphylum Vertebrata consists of seven extant classes (Vertebrata-classes). There are several arrangements of the classes, not all resulting in groups that possess natural taxa. The most convenient arrangement is into two superclasses; the superclass Pisces (fishes in the broadest sense including class Agnatha, class Elasmobranchiomorpha and class Osteichthyes) versus the superclass Tetrapoda (terrestrial tetrapods including class Amphibia, class Reptilia, class Aves and class Mammalia). One alternative is the Agnatha, or jawless fishes, versus the Gnathostomata, or jawed vertebrates. Another is the arrangement into the Amniota (including the reptiles, birds, and mammals) based on the possession of a complex set of extra-embryonic membranes, as opposed to the Anamniota (including the fishes and amphibians), which have only the trilaminate yolk sac.

More developed vertebrates (such as gnathostomes) possess jaws with teeth, paired fins with an internal skeleton articulating with limb girdles, lungs and bony scales. The tetrapods have limbs evolved from fins, digits, a neck, specialization of the segments of the vertebral column, and horny scales, hairs, or feathers.